Why does the buzz of a fly and a wasp differ?

It might look like a wasp, but this is actually a hoverfly

The buzzing sound of a fly or wasp is created by the vibration of the insect’s wings. A common housefly flaps its wings 200 times per second. That means it completes a flapping cycle – wing up, wing down, wing up – 200 times every second. That translates into a frequency of 200 Hertz. The human ear interprets frequency as pitch. For example, middle ‘C’ on the piano vibrates with a frequency of 261.6 Hertz. The higher or lower the frequency, the higher or lower the pitch. Four-winged insects like wasps and bees flap their wings at a slower frequency than two-winged flies, resulting in a deeper buzz.